I have ordered images from most of the online services, and MPIX quality is, in my opinion, the best that I have found. I only use the metallic paper with the luster finish for my prints, so I wouldn't be able to comment on the economy printing. The price is very fair in my opinion, not the cheapest, but cheaper than you would expect for the quality. I am a semi-professional photographer and do all of my own post processing, so I am not really interested in a lot of features on their website. I guess I can see how some people might not be completely satisfied with the site, as it is not as intuitive as some sites, but for my purposes it is easy to upload images and purchase prints.
In a nutshell, my review is based, not upon the website, but upon the quality of the work that they do as well as the turnaround time. To be able to place an order on Monday and get the images on Wednesday morning is fantastic. With expedited shipping at $12.59 I don't see how anyone could complain about that. I would not hesitate to recommend them to my closest friends and family...